Initial commit

.editorconfig

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+# Check out EditorConfig at: https://editorconfig.org
+
+root=true
+
+[*]
+charset=utf-8
+insert_final_newline=true
+max_line_length=100
+trim_trailing_whitespace=true
+
+[*.go]
+indent_size=2
+indent_style=tab
+
+[*.md]
+indent_size=unset
+indent_style=space
+
+[LICENSE]
+indent_size=unset
+indent_style=space
+max_line_length=80

.gitattributes

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+* text=auto
+
+# don't diff machine generated files
+go.sum -diff

.gitignore

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+gha.sum
+
+## IDEs
+.idea/
+.vscode/

LICENSE

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NOTES.md

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+# SHAttered git commits
+
+## Overview
+
+1. The [SHAttered website](https://shattered.io/) states it's possible to make malicious git commits with the same SHA as an existing commit.
+2. However, "the attack required 'the equivalent processing power of 6,500 years of single-CPU computations {...}.'" So it's not all that feasible in practice
+3. From [SHA1 on Wikipedia](https://en.wikipedia.org/wiki/SHA-1), a more recent attack requires approximately the same amount of computation "{...} at the time of publication would cost US$45K per generated collision.
+
+## Attack Description (in detail)
+
+### Steps
+
+1. Create valid commit and advertise it as a release (e.g. with a git tag or GitHub release).
+2. Get people to use it.
+3. Craft a malicious commit with the same hash.
+4. People using that release now use malicious code which they can't tell from the integrity hash.
+
+### Discussion
+
+- I don't think that the older the release is (in terms of descendant commits) the harder this attack becomes. Namely, the child of a commit's only "chaining" to the parent is the parent's commit hash. So, since the commit hash didn't change there's no need to find a collision for more than one commit. The implication is that this attack can be carried out for any historical commit.
+- Open questions:
+  - Can existing clones be used to detect this attack? What is git's behavior locally when the hash of a historic commit is unchanged but the contents did change?
+  - Can forks be used to detect this attack?
+
+### Impact
+
+- GitHub Actions
+  - Dependencies in GitHub Actions may be specified/identified/versioned by the commit SHA of the git repository that hosts the dependency (referred to as an "Action"). This is the most secure approach, alternatively it can be specified/identified/versioned using git refs (such as branches or tags) but those are inherently mutable.
+  - Hence, as a result of the attack GitHub Actions users that use external Actions have no guarantee the code that ran yesterday is the code that ran today.
+  - The only available protection is to limit what Actions that are allowed to be used in a project to Actions with a good security hygiene.
+- Go mod
+  - Reference: <https://go.dev/ref/mod#authenticating>
+- Dependencies in Go can (default, but alternatives exist) be pulled directly from a git repository. While you may reference the version you want to use by the commit hash,   the Go module system will actually compute a SHA256 hash over the source and use that in the `go.sum` file instead. It is this hash that's used for integrity purposes
+  - Thus, even if a malicious commit is created for an existing commit hash, the Go module system would detect this through the SHA256 hash of the source code.
+- Deno
+  - Dependencies in Deno can (^alternatives exist) be pulled directly from a git repository. As far as I can tell it doesn't rely on the commit hash for integrity ([ref](https://docs.deno.com/runtime/manual/basics/modules/integrity_checking)) and it seems to be using SHA256 ([ref](https://github.com/denoland/deno_lockfile/blob/75e3b2800e3fd6f3d62478a6cf15fd030d91a363/src/lib.rs#L29)), though I'm unsure what it hashes.
+
+## Outcome
+
+- Deno and Go are unaffected. Only the GitHub Actions ecosystem is affected.
+- Build a system for GitHub Actions that stores and compares better hashes of the Actions source code (i.e. repository). Hashes could be stored in the repository and checked as a first step of any job.
+  - <https://github.com/ericcornelissen/ghasum>

README.md

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+# `ghasum`
+
+Checksums for GitHub Actions.
+
+## Motivation
+
+The dependency ecosystem for GitHub Actions is fully reliant on git. Dependencies can be specified
+by a git ref (branch or tag) or commit SHA. Git refs provide no integrity guarantees. Commit SHAs do
+provide integrity guarantees, but since they're based on SHA1 these guarantees are weaker then
+necessary (see for example the [SHAttered] attack).
+
+Hence, this project aims to provide a way to get, record, and validate checksums for GitHub Actions
+dependencies using a more modern cryptographic hashing algorithm.
+
+## TODO
+
+An incomplete list of things that should be done:
+
+- [ ] Better user experience
+- [ ] Improve source code and use automated testing
+- [ ] At least a basic continuous integration setup
+- [ ] Record some sort of version in the checksum file in order to aid in changing the format or
+      changing the hash algorithm in the future
+- [ ] Convenient support to run as a GitHub Action to validate dependency integrity before the (rest
+      of the) job runs
+
+[shattered]: https://shattered.io/

go.mod

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+module github.com/ericcornelissen/ghasum
+
+go 1.21.0
+
+require github.com/go-git/go-git/v5 v5.9.0
+
+require (
+	dario.cat/mergo v1.0.0 // indirect
+	github.com/Microsoft/go-winio v0.6.1 // indirect
+	github.com/ProtonMail/go-crypto v0.0.0-20230828082145-3c4c8a2d2371 // indirect
+	github.com/acomagu/bufpipe v1.0.4 // indirect
+	github.com/cloudflare/circl v1.3.3 // indirect
+	github.com/cyphar/filepath-securejoin v0.2.4 // indirect
+	github.com/emirpasic/gods v1.18.1 // indirect
+	github.com/go-git/gcfg v1.5.1-0.20230307220236-3a3c6141e376 // indirect
+	github.com/go-git/go-billy/v5 v5.5.0 // indirect
+	github.com/golang/groupcache v0.0.0-20210331224755-41bb18bfe9da // indirect
+	github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99 // indirect
+	github.com/kevinburke/ssh_config v1.2.0 // indirect
+	github.com/pjbgf/sha1cd v0.3.0 // indirect
+	github.com/sergi/go-diff v1.1.0 // indirect
+	github.com/skeema/knownhosts v1.2.0 // indirect
+	github.com/xanzy/ssh-agent v0.3.3 // indirect
+	golang.org/x/crypto v0.13.0 // indirect
+	golang.org/x/mod v0.12.0
+	golang.org/x/net v0.15.0 // indirect
+	golang.org/x/sys v0.12.0 // indirect
+	golang.org/x/tools v0.13.0 // indirect
+	gopkg.in/warnings.v0 v0.1.2 // indirect
+	gopkg.in/yaml.v3 v3.0.1
+)